Currently, digital images are derived through various devices including digital cameras and scanners for digitally scanning of film images. Many times the sharpness of an image is degraded by optical elements or by irregularities in the image sensor. For these reasons, it is often desirable to sharpen an image after it has been converted to a digital form. Conventional sharpening methods, such as unsharp masking, achieve the appearance of edge sharpening by locally lightening the lighter portion of an edge region and locally darkening the darker portion of an edge region. The resulting increase in contrast provides the sharpening effect. Such methods can be applied to black and white digital images as well as to colored digital images.
Shown in FIG. 1A is a one-dimensional edge profile, which is a graph of image pixel code values plotted as a function of their position on a line running perpendicularly across an edge feature in a digital image. Higher code values correspond to lighter shades and lower code values to darker shades. In FIG. 1B, the same edge profile has been further blurred in accordance with the prior art technique of unsharp masking. The curve in FIG. 1B is subtracted from the curve in FIG. 1A and the resulting curve shown in FIG. 1C in which the amplitudes P and N, for positive and negative boost respectively, are approximately the same size. The difference curve of FIG. 1C is added to the original curve in FIG. 1A and this final curve, shown in FIG. 1D, depicts the profile of the sharpened edge. Although unsharp masking was originally a film technique, it also has a digital version. Shown in FIGS. 2A-C are examples of boost kernels which, when applied to a digital image, directly produce boost values analogous to those shown in FIG. 1C and sharpened edges analogous to those shown in FIG. 1D and shows the true edge contour, labeled (T), and the two false contours, labeled (F). The light and dark ridges are labeled (L) and (D) respectively.
When these conventional sharpening methods are applied too aggressively, image artifacts will appear. One recurring artifact is a false secondary edge contour which runs parallel to the true edge. This artifact can often be seen on both the lighter and darker sides of an edge that has been enhanced. The cause of the false contours for a single color channel is shown in FIG. 3 using a one dimensional edge profile. The enhanced edge profile follows the profile of FIG. 1D and shows the true edge contour, labeled (T), and the two false contours, labeled (F). The light and dark ridges are labeled (L) and (D), respectively.